#include<bits/stdc++.h>
#define ll long long
#define ld long double
#define mid ((l+r)>>1)
using namespace std;
namespace IO{
    char buff[1<<21],*p1=buff,*p2=buff;
    char getch(){
        return p1==p2&&(p2=((p1=buff)+fread(buff,1,1<<21,stdin)),p1==p2)?EOF:*p1++;
    }
    template<typename T>
    void read(T &x){
        char ch=getch();int fl=1;x=0;
        while(ch>'9'||ch<'0'){if(ch=='-')fl=-1;ch=getch();}
        while(ch<='9'&&ch>='0'){x=x*10+ch-48;ch=getch();}
        x*=fl;
    }
    template<typename T,typename ...Args>
    void read(T &x,Args& ...args){
        read(x);read(args...);
    }
    char obuf[1<<21],*p3=obuf;
    void putch(char ch){
        if(p3-obuf<(1<<21))*p3++=ch;
        else fwrite(obuf,p3-obuf,1,stdout),p3=obuf,*p3++=ch;
    }
    void pc(const char *s){
    	while(*s)putch(*s),s++;
    }
    char ch[100];
    template<typename T>
    void write(T x){
        if(!x)return putch('0');
        if(x<0)putch('-'),x*=-1;
        int top=0;
        while(x)ch[++top]=x%10+48,x/=10;
        while(top)putch(ch[top]),top--;
    }
    template<typename T,typename ...Args>
    void write(T x,Args ...args){
        write(x),putch(' '),write(args...);
    }
    void flush(){fwrite(obuf,p3-obuf,1,stdout);}
}
using namespace IO;
const int N=1e6+5;
int n,k;
struct node{
	int a,b;
}P[N];
ld p[N],q[N];
ld f[N][2],g[N][2];
signed main(){
	freopen("sleep.in","r",stdin);
	freopen("sleep.out","w",stdout);
	read(n,k);
	for(int i=1;i<=n;i++)read(P[i].a,P[i].b);
	sort(P+1,P+n+1,[](node A,node B){return 1ll*A.a*B.b>1ll*B.a*A.b;});
	for(int i=1;i<=n;i++)
		p[i]=(ld)P[i].a/P[i].b,
		q[i]=(ld)(P[i].b-P[i].a)/P[i].b;
	f[0][1]=1;
	for(int i=1;i<=k;i++){
		f[i][1]=(ld)f[i-1][1]*p[i];
		f[i][0]=(ld)(f[i-1][1]+f[i-1][0])*q[i];
	}
	g[n+1][0]=1;
	for(int i=n;i>=n-k+1;i--){
		g[i][0]=(ld)g[i+1][0]*q[i];
		g[i][1]=(ld)(g[i+1][0]+g[i+1][1])*p[i];
	}
	ld ans=0;
	for(int i=0;i<=k;i++){
		ld sum=(f[i][0]+f[i][1])*g[n-(k-i)+1][0]+f[i][1]*g[n-(k-i)+1][1];
		ans=max(ans,sum);
	}
	ans=1-ans;
	printf("%.9Lf",ans);
	return 0;
}

